Which particle is used to initiate nuclear fission in a reactor?

Option C is correct: Slow neutrons have the right energy level to be captured efficiently by U-235 nuclei, making them unstable and causing fission. Option A is incorrect: Protons carry positive charge and are repelled by the positively charged nucleus due to electrostatic repulsion, making them ineffective for initiating fission. Option B is incorrect: Alpha particles are helium nuclei with +2 charge and are also repelled by the nucleus due to Coulomb repulsion, making them unsuitable. Option D is incorrect: Beta particles are fast-moving electrons emitted during radioactive decay; they do not initiate nuclear fission. A moderator (heavy water or graphite) is used in reactors to slow down fast neutrons to thermal speeds required for sustained fission.